Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion (“Li-ion”) batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (measured in Wh/kg) and energy density (measured in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.
When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity-increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, and Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials including BiF3 (303 mAh/g, lithiated) and FeF3 (712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira, Fluoride based electrode materials for advanced energy storage devices. Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
Thus the advantage of using a Li metal negative electrode (sometimes referred to as an anode) is the much higher energy density of the entire cell, as compared to cells with graphitic or other intercalation negative electrode. A disadvantage of using pure Li metal is that lithium is highly reactive. By way of example, FIG. 1 is a schematic of an electrochemical cell 10 including an anode 12 including a form of lithium, a cathode 14, and a separator 16 in a fully charged state. A tab 18 provides electrical connection to the anode 12 while a tab 20 provides electrical connection to the cathode 14. As the cell 10 discharges, the lithium in the anode 12 moves to the cathode 14 resulting in the configuration of FIG. 2 in a discharged state. In FIG. 2, the anode 12 has very little lithium remaining.
In the ideal case the plating and stripping of the Li metal would be completely uniform such that the thickness of the Li metal across the length of the cell would be uniform. In practice the current density in the cell is non-uniform. One reason for this is the tabs 18 and 20 that distribute the electronic current across the electrode surfaces create electronic pathways of different lengths, thereby encouraging a non-uniform current distribution. Another reason is that temperature gradients create non-uniform local resistance to the flow of current, which will then distribute according to the lowest resistance pathway and in a non-uniform manner.
The result of a non-uniform current density is a non-uniform thickness of Li metal, which is shown graphically in FIG. 3. In FIG. 3, the Li metal is thicker at either end of the cell and thinner in the middle. The specific shape shown in FIG. 3 is meant to be indicative of the types of shape change that can occur, but other shapes are also recognized, including shapes in which one side is thicker than the other, and so on. The changing thickness of the Li metal may also lead to a changing thickness of other cell layers, including the separator and the cathode layers. In addition, while FIG. 3 shows all of the cell layers maintaining coherent interfaces, the layers may also partly separate as a result of shape changes, which in general is deleterious to cell performance.
Additionally, the lithium metal has a propensity to undergo morphology changes, which cause structures having a high surface area to form on and around the negative electrode when the cell is being charged. Exemplary high surface area structures include dendrites and mossy structures.
Dendrites are the most common failure mode for cells with Li metal anodes. The dendrites form with a needle-like structure and can grow through the separator during charging of the cell, resulting in an internal short. “Soft shorts” that burn out rapidly result in a temporary self-discharge of the cell, while “strong shorts” consisting of a higher, more stable contact area can lead to complete discharge of the cell, cell failure, and even thermal runaway. While dendrites typically grow through the separator during charge, shorts can also develop during discharge depending on the external pressure placed on the cell and/or internal volume changes that occur in both the negative and positive electrodes.
Because Li metal is highly electronically conductive, the surface of the Li tends to roughen as the metal is plated and stripped. Peaks in the surface grow as dendrites during charge. During discharge, some smoothing of the dendrites occurs. Nonetheless, there is typically some roughness that remains at the end of discharge. Depending on the depth of discharge, the overall roughness can be amplified from one cycle to the next. Because the metal is essentially at the same electrochemical potential throughout, potential and, to a lesser extent, concentration gradients in the electrolyte phase drive the change in morphology.
What is needed, therefore, is a battery system that reduces the potential for dendrite formation and the undesired morphological changes in the anode of battery cells having metal anodes.